1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by an electrophotographic process with laser light, and to a method for controlling the image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as laser beam printers and copying machines employing electrophotographic methods have been conventionally known. Such image forming apparatuses form images by scanning image bearing members with laser beams. Generally, an image forming apparatus employing an electrophotographic method forms an image by a plurality of processes such as charging, exposure, development, transferring, fixing, and cleaning processes.
A general electrophotographic process is performed as follows. First, a charging unit uniformly charges a photosensitive member serving as an image bearing member. Then, an exposure unit exposes the photosensitive member to a laser beam according to an image signal to form an electrostatic latent image on the photosensitive member. Herein, a direction in which the photosensitive member is scanned with the laser beam is referred to as a main scanning direction. The electrostatic latent image is formed while the photosensitive member is being rotated sequentially in a sub-scanning direction perpendicular to the main scanning direction. Subsequently, a developing unit develops the electrostatic latent image on the photosensitive member, and forms a toner image on the photosensitive member. Herein, the developing unit charges a toner, and supplies the charged toner to the photosensitive member using a developing roller that rotates at a substantially constant speed. The developing unit attaches the toner to the electrostatic latent image, thereby forming the toner image. The toner image on the photosensitive member is transferred to a recording medium and is fixed thereon. Accordingly, an image is formed on the recording medium. A cleaning unit collects a transfer residual toner remained on the photosensitive member.
In such an image forming apparatus, there are cases where horizontal stripes (hereinafter referred to as banding) caused by difference in density are generated on an image due to various causes. Such banding significantly deteriorates image quality.
For example, banding occurs when a variation in rotation speed of a photosensitive member generates a variation in main scanning interval (hereinafter referred to as a scanning interval) on a photosensitive member to be scanned by an exposure unit. When the rotation speed of the photosensitive member is high, the scanning interval increases. Thus, an amount of exposure per unit area decreases. If an image is formed in such a situation, a density of the image will be lower than that of an image that is originally expected. When the rotation speed of the photosensitive member is low, on the other hand, the scanning interval decreases, so that an amount of exposure per unit area increases. If an image is formed in such a situation, a density of the image will be higher than that of an image originally expected.
In addition, there are cases where banding occurs by variation in rotation speed of a developing roller. When the rotation speed of the developing roller is low, an amount of toner to be supplied decreases. Consequently, an amount of the toner to be adhered to an electrostatic latent image decreases, and a density of an image to be formed will be lower than that of an image originally expected. When a rotation speed of the developing roller is high, on the other hand, an amount of the toner to be supplied increases. Consequently, an amount of the toner to be adhered to an electrostatic latent image increases, and an imaged to be formed will have a higher density than that of an image originally expected.
Accordingly, Japanese Patent Application Laid-Open No. 2007-140402 discusses a method for correcting an exposure amount based on a result of reading an image formed on an intermediate transfer belt.
However, occurrences of banding differ depending on a gradation of an image to be formed. Although the method discussed in Japanese Patent Application Laid-Open No. 2007-140402 corrects the banding based on a result of reading an image having a specific gradation, the correction is not made in consideration of a gradation. Consequently, an appropriate correction cannot be made according to the gradation of an image to be formed, and the banding remains on the image.
Moreover, banding characteristics of respective gradations differ depending on a component (hereinafter referred to as a module) such as a photosensitive member or a developing member that is needed for image formation but a cause of the banding. Accordingly, an appropriate banding intensity according to an input gradation cannot be calculated without determination of which module causes the banding, or an appropriate correction cannot be made.